Cellulose acetate, particularly cellulose acetate having a degree of acetyl substitution of 2.6 or more (generally classified into cellulose triacetate), is tough and excellent in heat-resistance. Therefore, cellulose acetate has been used in various technical fields. For example, a cellulose acetate film is a representative photographic support. Further, a cellulose acetate film shows an excellent optical isotropy. Accordingly, a cellulose acetate film is used in a liquid crystal display, which has recently extended its market. In the liquid crystal display, the cellulose acetate film is used as a protective film of a polarizing plate, a phase retardation film or a color filter.
A cellulose acetate film is generally formed according to a solvent cast method, in which a cellulose acetate solution (dope) is cast onto a support, and dried to evaporate the solvent to form a film.
In preparation of a cellulose acetate product such as the film according to the solvent cast method, a degree of acetyl substitution of the cellulose acetate is closely related to a solubility of the cellulose acetate in a solvent and physical characteristics (including optical characteristics) of the products. It has been well known that cellulose acetate having a high degree of acetyl substitution shows a low solubility in a solvent, but forms a product of excellent physical characteristics. On the other hand, cellulose acetate having a low degree of acetyl substitution shows a high solubility in a solvent, but forms a product having problems in physical characteristics.
Dichloromethene is the most conventional solvent of cellulose acetate. Cellulose acetate is well dissolved in dichloromethane. Therefore, cellulose acetate having a high degree of acetyl substitution has been dissolved in dichloromethane to prepare a cellulose acetate product having excellent physical characteristics according to a conventional solvent cast method. The low solubility of the cellulose acetate having a high degree of acetyl substitution cannot be a problem where dichloromethane is used as the solvent.
However, use of hydrocarbon halides such as dichloromethane has recently been restricted severely to protect the global environment. Further, dichloromethane is apt to vaporize in the process for the preparation of a product, because it has a low boiling point (41° C.). Accordingly, dichloromethane may cause problems in the working environment. Therefore, the process is conducted under closed conditions.
Japanese Patent Provisional Publication Nos. 9(1997)-95544, 9(1997)-95557, 9(1997)-95538 propose a process for preparing the cellulose acetate solution, which comprises the steps of cooling a mixture of cellulose acetate and an organic solvent, and then heating the mixture to prepare the solution. The process comprising the cooling and heating steps (which is sometimes referred to as a cooling dissolution method) makes it possible to prepare a solution from cellulose acetate and an organic solvent in which cellulose acetate cannot be dissolved according to a conventional process. Therefore, the cooling dissolution method is very advantageous particularly where a film is prepared from cellulose triacetate (having a degree of acetyl substitution of 2.6 or more), which has poor solubility.